1. Field of the Invention
The present invention relates to the measurement of the intensity of a radiative flux, more particularly a thermal radiation emitted or received by a surface or wall so as to measure in particular the net thermal flux lost or gained by said wall or surface and, possibly, a convective flux.
It is known that radiative energy transfers play a very important role in numerous industrial mechanisms, in particular in the heating of premises.
In such mechanisms, the net thermal flux Fn lost or gained by a surface or wall is equal to the algebraic difference between the radiative flux Fe which it emits and the radiative flux Fa which it absorbs: Fn=Fe-Fa.
The net flux Fn takes into account the whole of the radiative exchanges and it is positive if the surface or wall emits more thermal radiation than it absorbs, whereas Fn is negative if the surface or wall absorbs more thermal radiation than it emits.
If we desire to maintain the temperature of a surface or wall constant, this surface or wall must be provided with a heating power equal to Fn if Fn is positive, or on the contrary remove therefrom heating power equal, in absolute value, to Fn if Fn is negative.
In practice, the radiative exchanges are measured between two surface or walls brought to different temperatures T1 and T2, or of a surface or wall which is at a temperature T1 and which is subject to a radiant temperature T2 representative of the radiative exchanges with the whole of the environmental medium.
Depending on the sign of the difference T1-T2 a net radiative flux is emitted by the surface or wall at temperatures T1 or is received thereby.
The purpose of the invention is precisely to provide a device for measuring the intensity of a radiative flux emitted or received by a surface or wall, which device is exact, accurate and sensitive, has a reduced time constant and is adapted to be applied to a surface or wall.
Generally the intensity of a radiative flux is measured by converting this radiative flux into a temperature difference in a device which receives this radiative flux, which converts this flux into a temperature difference as a function of this flux and which measures this temperature difference.
For this, the following may in particular be used:
an uninterrupted succession of two types of thermoelectric elements, of different thermoelectric powers, alternating and connected in series, thus forming a succession of successive thermoelectric couples with a series of thermoelectric junctions between two successive thermoelectric elements of different thermoelectric powers;
two types of coatings disposed alternately on the successive thermoelectric junction of the series of junctions, the coatings of the first type having a high absorbing power (so a high emissive power e1), whereas the coatings of the second type have a high reflecting power (so a low emissive power e2), with respect to the radiative flux F to be measured, so as to convert the radiative flux into a temperature difference between two successive thermoelectric junctions, while creating hot sources at the level of the junctions covered with coatings of the first type and cold sources at the level of alternate junctions covered with coatings of the second type, the hot sources receiving thermal energy proportional to e1F and the cold sources thermal energy proportional to e2F, the temperature difference between a hot source and an adjacent cold source being proportional to the intensity of said radiative flux F; and
means for measuring the sum of the elementary electromotive forces (e.m.f) produced by the elementary thermoelectric couples connected in series and each formed by a pair of successive thermoelectric elements of different thermoelectric powers and each including such a hot source and such a cold source, each elementary e.m.f being representative of said temperature difference and therefore of the intensity of said radiative flux.
2. Description of the Prior Art
Such a device is described for example in the U.S. Pat. No. 3,267,727, issued on the 23 Aug. 1966 to Mr. Theodore H. Benzinger.
In the device of this patent there is modulation of the incident radiant flux by the spatially variable emissivuty (high for coatings of the first type, but low for coatings of the second type) and this spatial variation of the radiative exchanges causes a variation of the surface temperature in the succession of series connected elementary thermocouples, while creating alternate hot sources and cold sources at the level of the different emissivity coatings.
Such a device has a relatively reduced sensitivity, of the order of 100 .mu.V per watt of radiative flux received and per dm2 of surface exposed to the radiative flux.
In order to increase the sensitivity of this type of radiative flux detector it is advantageous to increase the temperature difference between the hot sources and the cold sources, more especially by providing as high an emissivity difference as possible between the two types of coatings (of the first and of the second type respectively). A plastic insulating sheet of sufficient thickness could also be disposed between the thermopile and its support for increasing said temperature difference.
But such an increase in the temperature difference has drawbacks. In fact, the temperature differences between the hot sources and the cold sources tend to produce convective thermal exchanges between these two kinds of sources, these convective exchanges (which are all the higher the greater the temperature difference between the hot sources and the cold sources) generating in their turn surface temperature variations, which results in each said elementary thermocouple being subjected, between its hot sources and its cold sources, not only to the temperature difference (which it would be necessary to determine) resulting from the radiative flux to be measured, but also to a temperature difference resulting both from this flux and from said convective exchanges; thus, the temperature difference between each hot source and each cold source is no longer exactly proportional to the radiative flux and the same goes for the elementary e.m.f generated by each thermocouple. Thus, the sum of the elementary e.m.f.s connected in series is no longer an exact measurement of the radiative flux which it is designed to determine. Furthermore, the much higher convective exchanges (produced by ventilation for example) should also be taken into account which exist in the environment where the radiative flux measurement is being carried out.
To overcome these drawbacks, the invention proposes transforming the radiative flux to be measured directly into an electric voltage without passing through a temperature difference, so as to increase the sensitivity and reduce the above mentioned disturbances due to the convective thermal exchanges.
For this, using a chain of thermoelectric elements or elementary thermoelectric couples connected in series, which chain is formed by a continuous strip of a first conducting material covered in places with discontinuous deposits of a second conducting material having a thermoelectric power difference from that of the first material and a high electric conductivity;
each thermoelectric couple is unbalanced, by construction, in the longitudinal direction of the thermoelectric couple chain by creating, in this longitudinal direction, a spatial variation in the width of the thermoelectric element chain, namely the width of the continuous strip and/or of the deposits so as to make the thermoflux lines disymmetrical in the plane of this strip;
this strip is disposed on a thin support made from an insulating substance;
at least said deposits are covered with a high emissivity coating so that transformation of the radiative flux into heat localized in this high emissivity coating causes disymmetric tangential heat transfers in the thermoelectric element chain because of the presence of the electrolytic deposit of high thermal conductivity; and
the electromotive force is measured which is available between the two ends of this continuous strip, this electromotive force being proportional to the deflection of the thermal flux lines and so to the radiative flux which reaches said coating.